1. Technical Field
The present disclosure relates to a z-axis microelectromechanical device, having an improved stopper structure.
2. Description of the Related Art
Microelectromechanical devices (of a MEMS type), for example inertial sensors, accelerometers, gyroscopes, etc., base their operation on the presence of suspended structures made of semiconductor material, which are fixed to a substrate at one or more anchorage points via elastic suspension elements, and are mobile along one or more axes according to the configuration of the suspension elements. The suspended structures form one or more mobile masses that undergo displacements with respect to the substrate in the presence of external stimuli.
In particular, known z-axis MEMS devices include a mobile mass that is able to rotate out of a plane xy of main extension thereof about a fulcrum axis defined by corresponding suspension elements, originating a resultant movement along an axis z perpendicular to the plane xy. The mobile mass is unbalanced, in so far as it includes a first portion and a second portion, set on opposite sides with respect to the suspension elements and having a different twisting moment about the fulcrum axis. A stress (for example, an acceleration) directed in a direction perpendicular to the plane of the mobile mass causes a rotation (or tilting) thereof about the fulcrum axis, the quantity and direction of rotation correlated to the stress amount.
In a known manner, due to an external event, for example an impact or shock, the mobile masses of microelectromechanical devices can undergo undesirable displacements of considerable amount out of their plane xy of main extension, along the axis z, orthogonal to the plane xy. This displacement can, in the worst case, cause breaking of the suspension elements associated with the mobile masses or cause damage to the same mobile masses or other elements of the microelectromechanical device.
For this reason, stopper structures (generally known as “stoppers”) are provided in MEMS devices made with planar manufacturing processes; stoppers are designed to limit movements of the mobile masses and of the corresponding suspension elements and thus prevent damage.
Stopper structures are usually made as downward projections of a package or cap covers and englobes or encapsulates the microelectromechanical device. In particular, projections directed towards the mobile mass extend from an internal face of the package facing a top face of the mobile mass. These projecting portions of the package reduce the range of movement along the axis z for the mobile mass; in particular, the mobile mass stops, abutting against the projections, prior to onset of damage.
However, at its projecting portions, the package is close to the mobile mass, so that, if the mobile mass and package are kept at different potentials, an electrostatic interaction of a non-negligible amount is generated. This interaction can cause undesirable displacements of the mobile mass and in general malfunctioning of the microelectromechanical device. In detail, in the case of a sensor device, a displacement of the zero level (for example, an accelerometer 0 g level) in the presence of external electrical fields or a non-ratiometricity of the output signal can occur. This is rather frequent in so far as the package and consequently the projections coupled thereto are generally kept either at a floating potential, hence not at the same potential as the mobile mass or at a fixed potential, whilst the mobile mass is supplied with different potentials according to an operating condition of the device.
Other types of stopper structures have been proposed to overcome these drawbacks. For example, in the patent application No. WO-PCTIT0600576, filed on Jul. 26, 2006 in the name of the present applicant, a stopper structure is disclosed that includes a stopper element fixedly coupled to a mobile mass, and configured to abut against a stopper mass, facing the mobile mass in the plane xy, in response to a spurious movement along the axis z, in this way stopping the spurious movement. In detail, the stopper element is arranged on the opposite side of the stopper mass with respect to a direction of the spurious movement, and extends out of the space occupied by the mobile mass and into the space occupied by the stopper mass along the axis z.